1. Field of the Invention
The present invention relates to an electronic endoscope apparatus, and more specifically to an electronic endoscope apparatus capable of generating and displaying a spectroscopic image (picture) within a specific wavelength range by performing arithmetic operation on the image signals representing a color image.
2. Description of the Related Art
Recently, in the field of electronic endoscope systems with a solid state imaging device, those that performs spectroscopic imaging using combined narrow bandpass filters based on the spectral reflection factors at digestive organs, such as gastric mucosa and the like, i.e., electronic endoscope systems having therein narrow-band filters (Narrow Band Imaging, NBI) have been drawing attention. The system includes three narrow (wavelength) bandpass filters, instead of frame sequential type R (red), G (green), and B (blue) rotation filters. In the system, illumination light beams are sequentially outputted through the narrow bandpass filters, and three signals obtained by the illumination light beams are processed by changing the weighting thereof in the similar manner as in the R, G, B (RGB) signals, thereby a spectroscopic image is generated. According to such spectroscopic images, microscopic structures and the like, which were unable to be obtained in the past, may be detected in digestive organs, such as a stomach and a large intestine.
Unlike the frame sequential type endoscope system that uses narrow bandpass filters described above, the so-called simultaneous type endoscope system uses microscopic mosaic color filters which are disposed on the solid state imaging device. In the simultaneous type endoscope system, a method for generating a spectroscopic image based on the image signals obtained by imaging a observation target body while irradiating white light on the body is proposed as described, for example, in Japanese Unexamined Patent Publication 2003-93336, and a non-patent literature by Yoichi Miyake, University of Tokyo Press, 2000, pp 41, pp 147-153 currently available only in Japanese, and the title of which may be tentatively translated as “Analysis and Evaluation of Digital Color Image”). In the method, relationships between the respective RGB color sensitivity characteristics converted to numerical data, and spectroscopic characteristics of a specific narrow bandpass converted to numeral data are obtained as matrix data (a set of coefficients), and spectroscopic image signals are obtained by estimating a spectroscopic image which would be obtained through narrow bandpass filters by performing arithmetic operation between the matrix data and the RGB signals. Spectroscopic image generation through such arithmetic operation does not require a plurality of different filters corresponding to intended wavelength regions and replacement thereof, so that the system may be kept small with reduced cost.
In the mean time, in electronic endoscope systems, it is customary that the brightness of a displayed image is controlled within a desirable range to facilitate the observation of the image. Generally, such brightness control is performed through controlling the amount of light irradiated on the observation target body from the light source as described, for example, in Japanese Unexamined Patent Publication No. 2000-253307, or through controlling the gain of an amplifier for amplifying image signals outputted from the imaging device as described, for example, in Japanese unexamined Patent Publication No. 11 (1999)-75112. More specifically, as described in the patent publications described above, a method in which the opening of the aperture diaphragm disposed in front of the light source is controlled based on the luminance information (e.g., average luminance value of all of the pixels in a single frame, or the like) provided by the image signals outputted from the imaging device, or a method in which the gain of the amplifier circuit is controlled based also on the luminance information is proposed.
The control for setting the brightness of a displayed image within a predetermined range is, of course, desirable for the electronic endoscope systems for obtaining spectroscopic images described above. Here, in this case also, it may be conceivable to control the amount of light irradiated on the observation target body, or to control the gain of the amplifier circuit for amplifying the image signals outputted from the imaging device based on the luminance information provided by the image signals outputted from the imaging device. Contrary to the expectation, however, it has been found that such control method decreases the brightness of the spectroscopic images.